UFC 3 -430-11
14 February 2001
CHAPTER 5
LOCAL DEVICES AND INSTRUMENTATION
5-1
GENERAL. This chapter covers local instruments. Panel instruments are not
included. Instruments that are usually located on panels, such as controllers, are
covered in chapter 4.
Instruments should be located where they are accessible. Instruments that
must be operated during the start-up or shutdown o f equipment should be located as
close to the equipment as practical. Some instruments must be accessed continuously
for operation, others only during startup and shutdown. All instruments must be
accessible for calibration and maintenance. Locate instruments using the following
order of access preference.
Grade
Platform
Stairs
Ladder
Portable ladder
5-2
INDIVIDUAL ITEM REQUIREMENTS
5-2.1
Valves. This paragraph covers control valves, pressure regulators and
solenoid valves. Each type is discussed below.
5-2.1.1 Control Valves. Common types of flow characteristics for control valves
include quick opening, linear, and equal percentage. Control valves with equal
percentage flow characteristics are specified for most applications. Control valves with
linear flow characteristics are hard to tune at low flow and should be avoided. Select
the flow characteristic to suit the application. Tolerance criteria for control valves are
defined by ISA-S75.11, Inherent Flow Characteristics and Rangeability of Control
Valves.
5-2.1.1.1 Design Checklist. There are many items to be considered in control valve
selection. Checklists are provided in most manufacturer catalogs. Critical items that
are sometimes overlooked include type of shutoff, shutoff pressure, line hydrotest
pressure and controllability at turndown conditions. Review all pertinent sizing and
selection information including accessories when selecting a control valve.
5-2.1.1.2 Construction. Use carbon steel body with stainle ss steel trim. Other
materials may be specified when required by unique conditions.
5-2.1.1.3 Sizing. Size control valves to absorb 30 to 50 percent of the total system
pressure drop.
5-1
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